1. Field of Invention
The present invention relates to an illuminator, a projection display device having the same, and a method for driving the same. More particularly, the invention relates to a projection display device having excellent image expression and an illuminator used therein.
2. Related Art
Information technology devices have greatly been developed, thus increasing a demand for display devices with high resolution, low power consumption, and low profile, and thus advancing research and development. Among them, liquid crystal displays are expected to be able to change optical characteristics by electrically controlling the alignment of liquid crystal molecules to thereby match the above-described needs. A known example of such liquid crystal displays is a projection display device (liquid crystal projector) that projects an image emitted from an optical system using a liquid-crystal light valve, on an enlarged scale, onto a screen through a projector lens.
The projection display device uses a liquid-crystal light valve as light modulation device. In addition, a digital micro mirror device (DMD) is also in practical use as light modulator. The conventional projection display devices of this type have the following problems:
Sufficient contrast cannot be provided because of light leakage and stray light occurring in various optical elements that construct the optical system, thus having a narrow display luminous range (dynamic range), and thus being inferior to existing TV monitors using a cathode ray tube (hereinafter, referred to as a CRT) in image quality and impression.
Even when an increase in image quality is intended by various image-signal processings, sufficient effects cannot be given because of the fixed dynamic range.
The solution to the problems of the projection display devices, briefly, the method for expanding the dynamic range, includes changing the intensity of light incident on the light modulation device (light valve) depending on the image signal. To that end, it is known in the art to provide light flux adjusting device (light control device) in front of a light source. See, for example, JP-A-05-066501.
As described above, the adaptive light control in which the light flux of illumination and the brightness of the display image are varied depending on the contents (for example, brightness) of the display image is effective in increasing the contrast or tone reproduction. However, it tends to cause deterioration of gradations and insufficient brightness in highlight areas. Therefore, it is effective for images having dark images as moving pictures, while it may not be effective for ones having light images as sportscasting, probably acting negatively by the occurrence of flickering.
The display characteristic of shadow areas is not so increased when the projection image is viewed in light environment that the contrast may be decreased because of insufficient brightness.